You're going to hate this answer, but it depends. Reddit has good CPC and engagement rates, so long as you have content or an application that appeals to that

You're going to hate this answer, but it depends. Reddit has good CPC and engagement rates, so long as you have content or an application that appeals to that crowd. (See Gabriel Weinberg's post on the topic). StumbleUpon has a more mass-market appeal, but relatively high cost-per-view with low engagement. Adwords is expensive unless you spend a long time honing in on niche keywords. Even then it is expensive. There was a great post on ASmartBear this morning (guest post) about how effective cold-calling can be. And that's essentially free (not counting your labor hours). Pud has a great write-up on this topic: http://pud.com/post/5239917032/users http://pud.com/post/5239917032/users This is something I struggle with too, so I'll be interested to see the other responses.